A traditional form of a boat hull is a displacement hull, which is characterized by a rounded bilge. Buoyancy, or lift, is generated by the amount of water the hull displaces as it moves through the water. Wave-making resistance, resulting from the formation of a bow wave, is typically the dominant form of drag for displacement hulls. As the speed of the boat increases, the length, height and speed of the generated bow wave increases as well. The wave-making resistance may increase exponentially until the wavelength of the bow wave is equal to the waterline length of the boat. At this point, the boat may be trapped climbing the trough of its own large bow wave, resulting in a virtual barrier to speed increase.
Planing hulls are designed to overcome displacement hull speed by skimming across the surface of the water. At lower speeds, wave-making resistance is still the dominant form of drag. At higher speeds, planing hulls are designed to generate hydrodynamic lift forces proportional to the speed of the boat. The total lift felt by the hull is a combination of the hydrodynamic lift and hydrostatic lift. Hydrodynamic lift is caused by the water passing over the planing surface. Hydrostatic lift is a function of the underwater volume of the hull. At lower speeds, planing hulls are primarily supported by buoyant, hydrostatic forces. As speed increases, hydrodynamic lift is generated and hydrostatic forces acting on the hull gradually decrease. As the boat transitions into a planing regime, the hull rises above its static flotation level and trims up by the bow, thereby reducing the wetted surface significantly. Hydrodynamic lift may continue to increase until the hydrostatic force felt by the hull is negligible, and the boat is fully planing.